93 regarding how combustion-related components should be designed, quality controlled and tested in order to be certifiable. That means the guidelines are not 100% clear as to what kinds of parts UAV manufacturers should be selecting and integrating into their higherendurance powertrains. It therefore falls onUAVmanufacturers to select components for the highest levels of fuel efficiency and safety assurance, while also somehowkeeping their costs in check tomake their aircraft affordable. Fortunately, theUAV industry hasmade considerable advances in storing fuel safely and efficiently, alongwith its delivery, monitoring and injection, which engineers are taking note of for the newgeneration of heavy-liftingUAVs aswell as the emerging urban air mobilitymarket. Fuel storage Onemight not automatically think of fuel tanks and bladders as playing amajor role in fuel efficiency, but great strides have beenmade in recent years in the design and manufacture of storage systems that are both lightweight and resistant to damage (and by extension to leaking), particularly thanks to innovations frommotorsport making their way into theUAVworld. The bladders canmake aircraft lighter, reducing their required fuel consumption, and preventing leaks naturally reduces fuel expenses over the vehicle’s lifetime. Bladders built from soft film materials make for light and flexible fuel storage, with some able to stretch by around 600% before failure and to compress by 50% depending on the geometry and the type of support foam used inside. Reinforced fabrics are better known as a bladder material than soft films, and offer better puncture strength and crash resistance, although as a tradeoff they are around 30% heavier than films and less flexible. However, in some new fuel bladders, soft films have been used as a primary constructionmaterial while incorporating reinforced fabrics in key areaswhere contact with sharp or harsh internal hull structural members is a concern. Taking the best of bothworlds in thisway optimises theweight and survivability of the bladders. Both the film and fabric are often made from polyurethanes, particularly an ester or ether depending on whether the fuel is a diesel or heavy fuel versus a gasoline with a two-stroke oil mix. Coatings in the fabric bladders are often made from nylon or Dyneema. These materials and their qualities can be traced back to motorsport’s governing body the Federation Internationale de l’Automobile, where standards for tensile tearing, punctures, seam strength and more have been specified for many years, although it is only in recent years that the benefits they could bring even to FAA-certified aircraft have started to be recognised. As well as improving weight efficiency and ruggedness, flexible bladders can also be stitched into geometries that conform to a wide range of compartment shapes in UAVs, such as wings or bulkheads, and potentially even with structural rods or spars running through hollows in the bladder shape. Although tanks made from hard materials might do the same, flexible bladders are again at less risk of impacts, breaks or cracks. Fuel systems | Focus Capacitive fuel level sensors provide reliable, high-resolution information on remaining fuel levels amid high slosh and vibration (Courtesy of Reventec) Great strides have been made in recent years in the design and manufacture of storage systems that are both lightweight and resistant to damage Uncrewed Systems Technology | June/July 2023
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